With the trend toward performance elevation in portable information appliances such as notebook type PCs, PDAs, and portable telephones, there is a demand for a small high-capacity power source. A direct-methanol fuel cell (DMFC) is increasingly expected to be a potent example thereof. A DMFC is a fuel cell in which electrical energy is directly taken out from the chemical reaction of methanol with oxygen, and has merits that it has a high theoretical volume energy density and can be continuously used with refueling.
In initial fuel cells of the solid polyelectrolyte film type, an ion-exchange membrane including a hydrocarbon resin produced by copolymerizing styrene and divinylbenzene was employed as an electrolyte film. This electrolyte film, however, had problems that it had low durability and necessitated complicated steps. In order to improve such durability, a crosslinked polymer obtained by crosslinking with a silane a polymer having sulfo and alkoxysilyl groups bonded to the main chain not through a benzene ring has been investigated as an electrolyte film for fuel cells (see patent document 1). However, this electrolyte film has a problem that its mechanical strength is insufficient, although chemically stable. Production by the radiation-induced graft polymerization method has also been investigated as a technique capable of improving durability as well as facilitating production (see, for example, patent documents 2 to 4).
Since a fluororesin excellent in mechanical strength and chemical stability can be used as a base for the solid polyelectrolyte film produced by the radiation-induced graft polymerization method, that technique is advantageous for improving the mechanical strength and chemical stability of an electrolyte film (long life), improving dimensional stability (low swelling), and reducing methanol permeability. However, when a polyfunctional polymerizable monomer such as divinylbenzene or bisacrylamide is used as a crosslinking agent, there is a problem that the solution gels during the grafting reaction. Although gel removal is easy in the production on a laboratory scale, there is a possibility that a considerable difficulty might arise in industrial-scale continuous production.
Furthermore, a technique has been investigated in which an alkoxysilane is infiltrated into a perfluororesin electrolyte film, followed by conducting hydrolysis/dehydrating condensation to precipitate silica in ion channels thereby reducing methanol permeability (see, for example, non-patent document 1). However, there is a possibility that a problem concerning stability might arise, since the silica has no chemical bonds with the electrolyte film.    Patent Document 1: JP-A-2001-283635    Patent Document 2: JP-A-2001-348439    Patent Document 3: JP-A-2002-313364    Patent Document 4: JP-A-2003-82129    Non-Patent Document 1: Journal of Applied Polymer Science, Vol. 68, 747-763 (1998)